This invention relates to a process for producing gaseous and liquid nitrogen with a variable proportion of liquid product by low-temperature separation of air in a distillation column system, said system being based on a single column rather than a connected double column.
Single-column processes are known for the production of nitrogen. In contrast to the double-column process, single-column system has only a high pressure column (the single column) and no conventional low-pressure column, the latter being normally operated with a reflux of a liquid nitrogen-containing stream and a feed of oxygen, both from the high pressure column. Nevertheless, the distillation column system of this invention may have additional columns beyond the single column, for example for obtaining ultra pure nitrogen or oxygen. Such additional columns do not require a liquid nitrogen reflux for the ultra pure oxygen column or an oxygen-containing stream for the nitrogen column.
The xe2x80x9cdistillation column systemxe2x80x9d comprises distillation columns that are connected to one another, but not the heat exchangers or machines such as compressors or expansion engines. In the simplest case, the distillation column system is formed exclusively by the single column.
xe2x80x9cOxygen-enrichedxe2x80x9d is defined here as a mixture of producer gases that has a higher oxygen concentration than air up to virtually pure oxygen. For example, oxygen-enriched fractions have an oxygen content of 25 to 90%, preferably 30 to 80%. (All percentages related here and below are molar percents, unless otherwise indicated.)
A single column distillation system including a nitrogen circuit is substantially disclosed in U.S. Pat. No. 4,400,188 as follows:
compressed air is cooled in a main heat exchanger, and fed to single column;
a nitrogen-rich fraction is drawn off from the distillation column system and is compressed, at least in part, in a circulation compressor;
a first part of compound nitrogen-rich fraction is fed downstream from the circulation compressor to a liquefaction chamber of a condenser-evaporator and is condensed therein under a pressure higher than the operating pressure of the single column, thereby forming a nitrogen-rich liquid;
a liquid, oxygen-enriched fraction is withdrawn from the distillation column system and is at least partially evaporated in an evaporation chamber of the condenser-evaporator;
a first oxygen-enriched gas is withdrawn from the evaporation chamber and introduced into the single column, as ascending vapor; and
a second part of the compressed nitrogen-rich fraction is withdrawn at least at times as gaseous nitrogen product.
A condenser-evaporator, which provides the bottom heating of the single column, is heated with the nitrogen which was brought to a level above column pressure in the circulation compressor. Process cold is produced by a conventional residual-gas turbine, which is operated with gas from another condenser-evaporator, a top condenser.
An object of the invention is to provide an energy-wise improved process of the above-mentioned type and a corresponding apparatus. Upon further study of the specification and appended claims, the object and advantages of the invention will become apparent.
To at least partially achieve these objects, a second oxygen-enriched gas is removed from the evaporation chamber of the condenser-evaporator, is machine expanded and is heated in the main heat exchanger.
The entire reflux liquid for the single column is preferably produced in the condenser-evaporator. In general, only a single condenser-evaporator is therefore necessary.
Air compressors and circulation compressors can be formed by a single machine, namely by a combi-machine, in which several pinion gears are arranged on a shaft, some of which form part of the air compressor and one or more form part of the circulation compressor.
The circulation compressor can be formed at least partially by a compressor that is coupled to the residual-gas turbine, whereby at least a portion of the mechanical energy that is produced in the machine expansion of the second oxygen-enriched gas is used for compression of the first portion and/or the second portion of the nitrogen-rich fraction.
In addition, the invention relates to a system comprising:
Apparatus for obtaining gaseous nitrogen by low-temperature separation from air with distillation column system comprising a single column (4), an air compressor, a main heat exchanger, passage means for feed air between the single column (4) from the air compressor through the main heat exchanger (2),
a circulation compressor (9,1063) for compression of the first portion of a nitrogen-rich fraction (5, 7, 8) from the distillation column system,
a circulation line (12, 13) from the outlet of circulation compressor (1063, 9) to a liquefaction chamber of a condenser-evaporator (14),
means for feeding a liquid, oxygen-enriched fraction from the distillation column system to the evaporation chamber of condenser-evaporator (14),
means for the production of a first oxygen-enriched gas (234, 533) from vapor (232) formed in the evaporation chamber of the condenser-evaporator (14) and for introduction into the single column (4) and a
a gas production line for drawing off a second portion (19, 20, 1064) of nitrogen-rich fraction (5, 7, 8) as a gaseous nitrogen product, said apparatus further comprising:
a machine (23) for engine expanding a second oxygen-enriched gas (221, 521) from the evaporation chamber of condenser-evaporator (14), and conduits leading from said evaporation chamber to said machine.